1. Field of the Invention
The present invention relates generally to polystyrene materials, and more particularly, to an apparatus and method for manufacturing expandable polystyrene (EPS) pellets. EPS pellets or beads are used in various applications, including loose fill packing materials and foam formed or molded plastic materials.
EPS material is lightweight, inexpensive, and may be molded into rigid shapes to fit a wide variety of applications, such as coffee cups, egg crates or packing inserts. Thus, for many applications, EPS materials provide a superior alternative to other packaging materials in terms of price and quality.
Recently, EPS materials also have been used for manufacturing bicycle safety helmets. These helmets are lightweight and highly impact absorbent.
As background, finished polystyrene products are formed by expanding small pellets of EPS material. In finished form, these expanded pellets are comprised of many small cells, and they are fused together to form a molded part.
2. Description of the Prior Art
Systems for manufacturing EPS materials are known. For example, EPS materials may be made in large batch processors. In these systems, a large amount of raw materials (plastics, gas, plasticizer, and the like) are simultaneously processed and gassified to form EPS pellets or beads. Generally, in these batch systems, the size of the pellets can be controlled by selecting certain processing conditions (e.g., time, temperature, pressure, ingredients, and the like). However, the resulting pellets are not uniform. Rather, they vary in size, within a range of sizes corresponding to a standard bell curve distribution.
It also is known to manufacture EPS materials using a continuous process extruder system. In these systems, a continuous supply of raw materials is input to the extruder, heated, and mixed with a blowing agent (e.g., pentane gas) and a plasticizer. A plurality of strands of EPS material then are drawn from the extruder through perforations in a die, and cut into pellets of EPS material. However, in known extruding systems, the strand dimensions (e.g., diameter) often vary from perforation to perforation of the die based on a number of factors, including the die configuration and the spacing and relative heights of the perforations. Also, pressure variations at the die plate can lead to clogging of the perforations, and to further variations in strand dimensions (e.g., diameter). Thus, conventional extrusion systems often produce EPS pellets of non-uniform size.
Finally, it is known that the texture and finish of polystyrene products may be improved by annealing extruded EPS pellets prior to expanding them. Specifically, EPS material has a knurled texture when it is extruded. Thus, for example, it is known to anneal EPS pellets by conveying the EPS pellets through a steam chamber on a conveyor belt, to give the EPS pellets a more uniform, smooth surface texture and finish. However, it is difficult to achieve uniform annealing of EPS pellets using this method. Moreover, even if uniform annealing is achieved, the size, and texture or finish of the expanded pellets still may vary if the size or finish characteristics of the initial, unannealed EPS pellets varies.